Polarization purity control device and gas laser apparatus provided with the same

ABSTRACT

A degree of polarization control device includes:
         a calcium fluoride crystal substrate for transmitting a laser beam;   a polarization monitor for measuring the degree of polarization of a laser beam transmitted through the calcium fluoride crystal substrate; and   a controller for controlling the rotation angle of the calcium fluoride crystal substrate according to the degree of polarization measured by the polarization monitor;   the calcium fluoride crystal substrate being formed by a flat plate having a laser beam entering surface and a laser beam exiting surface running in parallel with the (111) crystal face, the Brewster angle being selected for the incident angle, the rotation angle around the [111] axis operating as a central axis being controlled by the controller.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Japanese Patent Application No.2008-213529 filed in Japan on Aug. 22, 2008, the contents of which areincorporated by this reference.

BACKGROUND OF THE INVENTION

The present invention relates to a degree of polarization control deviceto be used in a semiconductor exposure apparatus having an excimer laseror a molecular fluorine laser and also to a gas laser apparatus providedwith the same.

(Exposure Light Source)

In the trend of making semiconductor integrated circuits finer and moreintegrated, an improved resolution is required for semiconductorexposure apparatus. To meet this demand, efforts are being paid to useshorter wavelength for the laser beam emitted from an exposure lightsource. Gas laser apparatus are being popularly employed as exposurelight sources in place of conventional mercury lamps. KrF excimer laserapparatus for emitting deep ultraviolet rays of a wavelength of 248 nmand ArF excimer laser apparatus for emitting vacuum ultraviolet rays ofa wavelength of 193 nm are being used as gas laser apparatus forexposure. Attempts are being made to apply a liquid immersion techniqueof reducing the apparent wavelength of an exposure light source byfilling the gap between an exposure lens and a wafer, thereby shiftingthe refractive index, to an ArF excimer laser apparatus as an exposuretechnique of the next generation. With ArF excimer laser liquidimmersion, the wavelength is reduced to 134 nm when immersed in purewater. F₂ laser liquid immersion exposure may possibly be adopted for F₂(molecular fluorine) laser apparatus that emit vacuum ultraviolet raysof a wavelength of 157 nm as exposure light sources of the thirdgeneration. The wavelength is believed to be made equal to 115 nm by F₂laser liquid immersion exposure.

(Exposure Optical Element and Chromatic Aberration)

Many semiconductor exposure apparatus adopt a projection optical systemas the optical system thereof. In a projection optical system, opticalelements such as lenses having different refractive indexes are combinedto correct the chromatic aberration. At present, optical materials thatare suitable as lens materials of projection optical systems for thewavelength (ultraviolet) range between 248 nm and 157 nm of lasersoperating as: exposure light sources are only synthetic quartz and CaF₂.For this reason, monochromatic lenses of the total refraction type thatare formed only by synthetic quartz are adopted as projection lenses forKrF excimer lasers, whereas partially achromatic lenses of the totalrefraction type that are formed by synthetic quartz and CaF₂ are adoptedas projection lenses for ArF excimer lasers. However, the naturaloscillation spectrum line width of both KrF excimer lasers and ArFexcimer lasers is as wide as about 350 pm to 400 pm so that, when such aprojection lens is used, chromatic aberration occurs to reduce theresolving power. Therefore, the spectrum line width of the laser beamsemitted from such gas laser apparatus needs to be narrowed to such adegree at which the chromatic aberration can be disregarded. For thisreason, a band narrowing module having a band narrowing element (etalon,grating or the like) is arranged in the laser oscillators of such gaslaser apparatus to realize band narrowing of the spectrum line width.

(Liquid Immersion Lithography and Polarized Light Illumination)

As described above, in the case of ArF excimer laser liquid immersionlithography, the refractive index will be 1.44 when H₂O is employed as amedium so that the lens numerical aperture NA that is proportional tothe refractive index can be theoretically increased to 1.44 times of theconventional numerical aperture. As the NA is increased, the influenceof the degree of polarization of the laser beam that is the light sourcewill increase. While there is no influence in the case of TE polarizedlight whose direction of polarization is parallel to the direction ofthe mask pattern, the image contrast will become low in the case of TMpolarized light whose direction of polarization is orthogonal to thedirection of the mask pattern. This is because the direction of theelectric field vector at the focal point on the wafer is different inthe latter case so that the intensity becomes weak as the incident angleto the wafer increases if compared with the former case where thedirection of the electric field vector is same and identical. Theinfluence thereof is intensified when the NA approaches or exceeds 1.0and ArF excimer laser liquid immersion falls into such a case.Therefore, a desired state of polarization needs to be controlled forthe illumination system of an exposure apparatus as described above. Tocontrol such polarized light illumination, the polarization of the laserbeam input to the illumination system of the exposure apparatus isrequired to be in a linearly polarized state. The degree of polarizationis the ratio of the linear polarization and the non-linear polarizationthat are measured and the polarization of a laser beam is required tomaintain a high degree of polarization. As illustrated in FIG. 20, whena polarizer is driven to rotate and the maximum value I max and theminimum value 1 min of the intensity of transmitted light are measured,the degree of polarization is expressed by the formula depicted below.

P=(Imax−Imin)/(Imax+Imin)  (1)

(Prior Art for Raising Polarization Purity)

The techniques described in Patent Document 1 and Patent Document 2 areknown as techniques for raising the degree of polarization of a laserbeam.

The technique described in Patent Document 1 provides a method ofpreventing degree of polarization from being degraded by intrinsicbirefringence that arises when light passes the inside of an opticalelement by making the optical axis of a laser beam to be transmittedperpendicularly relative to the (100) crystal face of the calciumfluoride crystal of the optical element used for a laser.

However, the above-described prior art has the following problem.

The degree of polarization of a laser beam is degraded by birefringenceof the optical element in the laser apparatus when the laser beam passesthrough the optical element. Birefringence includes stress birefringencecaused by external mechanical stress and/or thermal stress and intrinsicbirefringence that intrinsically exists and is expressed by the crystalstructure thereof if such stresses do not exist.

The technique described in Patent Document 1 is to prevent degradationof degree of polarization due to intrinsic birefringence by arranging alaser beam to pass perpendicularly relative to the (100) crystal face ofan optical element. Stress birefringence that arises when stress isapplied is largest in the direction that is perpendicular relative tothe (100) crystal face and, when it is used as a chamber window, stressbirefringence can possibly take place due to the stress that arises whenholding the window, the gas pressure of several atmospheric pressures inthe chamber and/or the stress caused by the thermal stress that arisesby laser beam irradiation.

Additionally, a cut surface is produced at an angle of 17.58° or 26.76°relative to the (111) crystal face and cut surfaces are used as theopposite surfaces of the chamber window so that the following twoproblems arise. One is that, since the surface coarseness of the cutsurfaces does not allow small high precision polishing to reduce thethreshold value for the surface damage caused by laser irradiation. Theother is that, when used as a chamber window, it is subjected to gaspressure of about 4,000 hPa so that it can be broken at the (111)crystal face that is apt to be cleaved. Furthermore, when the cutsurface is produced at 17.58° relative to the (111) crystal face, theangle formed by the chamber window and the optical axis is 70° and henceFresnel reflection of P-polarized light and that of S-polarized lightare 4.2% and 30.0% respectively so that, although the P-polarized lightcomponent is selected, the Fresnel reflection of P-polarized light islarge as a result of being transmitted through the window to make itimpossible to secure the laser output.

Thus, Patent Document 2 represents a technique of preventing degradationof degree of polarization due to intrinsic birefringence and stressbirefringence from taking place by means of an optical element for anultraviolet gas laser such as a window made of calcium fluoride crystaland having two faces, one of which, or face 2, is adapted to receiveultraviolet rays entering through it and exiting from the other face andat least one of which is parallel to the (110) crystal face of thecalcium fluoride crystal and also preventing cracks and defects fromarising by laser irradiation by smoothing the cut surfaces.

A technique of arranging a ½ wave plate and a polarizer on the opticalaxis and driving it to rotate in order to control the polarizationazimuth thereof and raise the degree of polarization has also beendisclosed (Patent Document 3).

-   [Patent Document 1]-   U.S. Patent Application Publication No. 2003/219056: Specification-   [Patent Document 2]-   JP-A-2006-73921-   [Patent Document 3]-   JP-A-2006-179600

However, the technique represented in Patent Document 2 can give rise tocleavage in operation by the chamber gas pressure that is appliedperpendicularly to the window and the mechanical stress for holding thewindow because it prevents degradation of degree of polarization due tointrinsic birefringence and stress birefringence and selects the (110)crystal face for a cut surface of the calcium fluoride crystal.Additionally, slippage can take place along the (111) crystal face inthe crystal and the window can be broken as a result.

The technique of Patent Document 3 raises the degree of polarization bypassing a polarizer that allows only p-polarized light to pass throughit so that the s-polarized component is removed by reflection and byshifting the optical axis.

Therefore, the intensity of light of the s-polarized component is lost.In short, a loss arises. The gas stress in the chamber needs to beraised and/or the discharge voltage needs to be increased in order tocompensate the loss. When the gas stress in the chamber is raised, theload to be borne by the chamber window due to an increase of gas stressand a fall of degree of polarization increases to shorten the servicelife of the apparatus. Additionally, when the discharge voltage isincreased, the load applied to the ps separation membrane and the lotionprism arranged on the optical axis representing a high light energydensity is increased to damage them within a short period of time.Furthermore, it is not possible to control the degree of polarization.

In view of the above-identified problems of the prior art, it istherefore an object of the present invention to provide a degree ofpolarization control device formed by using calcium fluoride crystal toreduce the damaging phenomenon caused by mechanical stress and gasstress, control the degree of polarization and, at the same time,suppress the degradation due to irradiation of a strong ultravioletlaser beam (ArF in particular) and also a gas laser apparatus providedwith such a degree of polarization control device.

SUMMARY OF THE INVENTION

In an aspect of the present invention, the above object is achieved byproviding a degree of polarization control device including: a calciumfluoride crystal substrate for transmitting a laser beam; a polarizationmonitor for measuring the degree of polarization of a laser beamtransmitted through the calcium fluoride crystal substrate; and acontroller for controlling the rotation angle of the calcium fluoridecrystal substrate according to the degree of polarization measured bythe polarization monitor, in which the calcium fluoride crystalsubstrate being formed by a flat plate having a laser beam enteringsurface and a laser beam exiting surface running in parallel with the(111) crystal face, the Brewster angle being selected for the incidentangle, the rotation angle around the [111] axis operating as a centralaxis being controlled by the controller.

Preferably, a degree of polarization control device as defined aboveincludes a plurality of calcium fluoride crystal substrates.

Preferably, a degree of polarization control device as defined aboveincludes a monitor module for observing the condition of the laser beam,the calcium fluoride crystal substrate being adapted to split the laserbeam in order to make it enter the monitor module.

In another aspect of the present invention, there is provided a gaslaser apparatus provided with a degree of polarization control device asdefined above, the apparatus including: a chamber; a laser gas containedin the chamber in a sealed condition; a means for exciting the lasergas; a window arranged at the chamber to allow the laser beam generatedfrom the excited laser gas to exit to the outside of the chamber; and adegree of polarization control device for controlling the degree ofpolarization of the laser beam exiting from the window.

Thus, the present invention can reduce the damaging phenomenon caused bymechanical stress and gas stress, control the degree of polarizationand, at the same time, suppress the degradation due to irradiation of astrong ultraviolet laser beam (ArF in particular).

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the feature of construction,combinations of elements and arrangement of parts which will beexemplified in the construction hereinafter set forth and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the crystal lattice of CaF₂;

FIG. 2 is a schematic illustration of the definition of angles θ and φof the light proceeding direction relative to the axis [001] and [100]of CaF₂ crystal;

FIG. 3 is a schematic cross-sectional view of a CaF₂ crystal substrate;

FIG. 4 is a schematic top view of a CaF₂ crystal substrate;

FIG. 5 is a schematic illustration of a state of polarization ofcrystal;

FIG. 6 is a schematic illustration of phase difference;

FIG. 7 is a schematic illustration of a state of polarizationobservation experiment system;

FIG. 8 is a schematic illustration of a measuring element;

FIG. 9 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the degree ofpolarization of an incident laser beam is made equal to 99.9%;

FIG. 10 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the degree ofpolarization of an incident laser beam is made equal to 97.1%;

FIG. 11 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the pulseenergy of incident light is varied;

FIG. 12 is a schematic illustration of the system according to a firstembodiment;

FIG. 13 is a schematic illustration of the polarization monitor of thesystem according to the first embodiment;

FIG. 14 is a flowchart illustrating Example 1 according to the firstembodiment;

FIG. 15 is a flowchart illustrating Example 2 according to the firstembodiment;

FIG. 16 is a schematic illustration of the system according to a secondembodiment;

FIG. 17 is a flowchart illustrating Example 1 according to the secondembodiment;

FIG. 18 is a flowchart illustrating Example 2 according to the secondembodiment;

FIG. 19 is a schematic illustration of the system according to a thirdembodiment; and

FIG. 20 is a schematic illustration of measurement of the intensity oftransmitted light.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Now, vacuum ultraviolet laser apparatus according to embodiments of thepresent invention will be described below.

FIG. 1 is a schematic illustration of the crystal lattice of CaF₂. Inthis embodiment, a CaF₂ crystal is cut along the (111) crystal face inaccordance with the crystal orientation. CaF₂ crystal is formed as aface-centered cubic structure as illustrated in FIG. 1.

As the angles θ and φ of the light proceeding direction L relative tothe axis [001] and [100] of CaF₂ crystal are defined in a manner asillustrated in FIG. 2, the direction of the [111] axis is by turndefined by φ=45° and θ=54.74° in FIG. 2. Since the surface of the (111)crystal face is harder than the surfaces of the other crystal axes andhence the hardest, the surface coarseness thereof is low and it can bepolished with few latent flaws.

Now, the state in which an optical axis L passes through CaF₂ crystal 3will be described below.

FIG. 3 is a schematic cross-sectional view of the CaF₂ (calciumfluoride) crystal 3 formed by using CaF₂ of this embodiment before beingrotated and FIG. 4 is a schematic top view of the CaF₂ crystal 3 afterbeing rotated.

FIG. 3 is a cross-sectional view taken along the face that includes the[001] axis, the [110] axis and the [111] axis. The CaF₂ crystal 3 ofCaF₂ is polished at the first surface 3 a and the second surface 3 bthat are parallel to the (111) crystal face. For example, a laser beamenters the CaF₂ crystal 3 at the center thereof with entering angleα=55.7° relative to the CaF₂ crystal 3 in the face that includes the[001] axis, the [110] axis and the [111] axis. Then, light is refractedwith angle of refraction β=33.4° at the first surface 3 a according tothe Snell's law. At this time, the CaF₂ crystal 3 is so arranged thatthe optical axis of refraction L in the inside of the CaF₂ istransmitted in the face that includes the [001] axis, the [110] axis andthe [111] axis and between the angle formed by the [111] axis and the[001] axis (0°≦γ≦54.7°). Then, the laser beam is transmitted through theCaF₂ crystal 3 and exits the CaF₂ crystal with exiting angle α=55.7° inthe face that includes the [001] axis, the [110] axis and the [111] axisat the second surface 3 b according to the Snell's law just like at thefirst surface 3 a.

In this embodiment, the CaF₂ crystal 3 is arranged at a position rotatedfrom this state by angle θ relative to the [111] axis that operates as acentral axis.

FIG. 4 is a schematic top view of a CaF₂ crystal 3 as viewed from rightabove the [111] axis. The face orientation axes of the CaF₂ crystal 3are depicted radially. The CaF₂ crystal 3 is formed as a face-centeredcubic structure as illustrated in FIG. 1. Thus, the axes of crystalorientation is threefold symmetrical when the axis is the axis ofsymmetry. Therefore, if the CaF₂ crystal 3 is viewed from right aboveand the [001] axis is selected as a reference axis, while the angle asviewed clockwise is negative and the angle as viewed counterclockwise ispositive, then the angle formed by the [001] axis and the [011] axis is−60° and the angle formed by the [001] axis and the [010] axis is −120°,whereas the angle formed by the [001] axis and the [110] axis is 180°and the angle formed by the [001] axis and the [101] axis and the angleformed by the [001] axis and the [100] axis are 60° and 120°respectively.

As illustrated in FIG. 4, the CaF₂ crystal 3 is arranged at a positionrotated by angle θ° around the [111] axis, which operates as a centralaxis, relative to the arrangement by which the laser beam entering thecrystal passes through the face that includes the [111] axis and the[001] axis.

Now, how the state of polarization changes due to birefringence will bedescribed below. Generally, light propagating through crystal is alinear combination of two waves in a linearly polarized state that areorthogonal relative to each other and the state of polarization and thedirection of polarization of each of them are determined as a functionof the phase velocity and the amplitude thereof. When birefringence isproduced in crystal by thermal stress, mechanical stress and/or someother stress, the phase velocity of a light beam propagating throughcrystal is shifted in the direction of polarization. As a result, whenlight propagating through crystal is split in the two directionsincluding the direction of the fast axis and that of the slow axis thatare orthogonal relative to each other, the phases of lights propagatingin the two directions are varied from each other because of thedifference of velocity of light (due to birefringence).

Because of this phenomenon, when linearly polarized light enters asillustrated in FIG. 5, it is turned into elliptically polarized light(circularly polarized light when the phase difference between the twocomponents is made equal to 90°) and exits in a changed state ofpolarization. The value of the phase difference due to birefringencevaries as a function of the crystal orientation in which lightpropagates. The phase difference is nil when light propagates in thecrystal orientation that operates as optical axis because nobirefringence takes place. When only intrinsic birefringence of CaF₂crystal is taken into consideration, the optical axis will be the [111]axis and the [001] axis, the axis, the [010] axis or an axis equivalentto it. If the direction of polarization of entering light is parallel orperpendicular to the fast axis or the slow axis of the crystal asillustrated in FIG. 6, light is not split into two components so that nophase difference arises.

A CaF₂ crystal was arranged for measurement with the incident angle oflight to be used in Chamber 1 and driven to rotate around the [111] axisthat operated as a central axis to change the crystal orientation forpropagation of light. Then, the change in the state of polarization wasobserved. Some of the obtained results will be described below.

FIG. 7 is a schematic illustration of a state of polarization stateobservation experiment system. A linearly polarized laser beam emittedfrom a narrow band ArF laser 101 (4 kHz, 10 mJ) was used and the CaF₂crystal 3 was arranged with an incident angle of α=55.7°. The directionof polarization of the entering laser beam was made to be in line withthat of the actual device and the laser beam was made to enter in adirection parallel to the sheet of FIG. 7. After passing through theCaF₂ crystal 3, the laser beam was made to enter a degree ofpolarization meter 103 to measure its linear degree of polarization. Inthe degree of polarization meter 103, two folding windows 104 a, 104 bare used in order to prevent the degree of polarization of the reflectedlaser beam from being shifted by folding the light path. The laser beamis made to pass a lotion prism 105 and the output thereof is observed bymeans of a sensor 106. The lotion prism 105 was driven to rotate whileobserving the output and the linear degree of polarization was measuredby means of the formula (1) described earlier. As illustrated in FIG. 8,the CaF₂ crystal was cut along the (111) crystal face and driven torotate around the [111] axis that operated as a center of rotation atintervals of 10° and the change of the linear degree of polarization wasmeasured over the range of 0° through 360°.

FIG. 9 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the degree ofpolarization of the incident laser beam is made equal to 99.9%. θ=0°refers to that the optical axis L is in the direction of the [001] axis.The positive direction of the rotation angle θ refers to that the CaF₂crystal 3 was driven to rotate counterclockwise.

As illustrated in FIG. 9, it was found that the degree of polarizationfalls at intervals of 60° or at θ=30°, 90° and 150°. Conversely, it wasfound that the degree of polarization does not change in the directionsof θ=0°, 60° and 120°. As a result, it was found that the phasedifference does not change when light propagates in the directions ofthe [001] axis and the [101] axis, whereas the phase difference changesat angles between them. It was also found that the phase differencechanges maximally at angles shifted from the [001] axis by 30°.

FIG. 10 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the degree ofpolarization of the incident laser beam is made equal to 97.1%. From theillustrated results, similarly, the degree of polarization of the laserbeam does not change at or near θ=0°, 60° and 120° but changes maximallyat or near θ=30°, 90° and 150° after the transmission of the laser.

Differently stated, when light propagates in the directions of the [001]axis and the [101] axis, the phase difference thereof does not change,although the phase difference changes at angles between them.Particularly, the phase difference changes maximally when the angleshift is 30° or 90° from the direction of the [001] axis. The degree ofpolarization is improved by a phase shift because the shift takes placein the direction of reducing the difference of the phase that has beenshifted originally as a result of passing through the crystal.

FIG. 11 is a graph illustrating the results of measurement of the lineardegree of polarization relative to the rotation angle when the pulseenergy of incident light is varied. It was also found that, when thepulse energy of incident laser beam is varied, the characteristicamplitude (the extent of phase difference) also varies. In other words,the extent of stress birefringence due to thermal stress varies.

For this reason, it was found that this characteristic is relative tostress birefringence and the directions of the [001] axis and the [101]axis are those of fast axes or slow axes (or optical axes) and hence,while the phase difference does not change when light is propagated inthese directions, the phase difference is maximized at the middlethereof, or at 30°.

FIG. 12 is a schematic illustration of the configuration of the opticalsystem of 2-stage laser system 1 and an example of arrangement of thefirst embodiment of degree of polarization control device according tothe present invention in the laser system.

The 2-stage laser system 1 includes an oscillator laser 10 and anamplifier laser 20 for receiving the laser beam (seed beam) oscillatedby the oscillator laser 10 and amplifies it. An ArF excimer laserapparatus or an F₂ laser apparatus particularly for exposures thatrequire a high output level not less than 40 W in a narrow band isadopted.

The oscillator laser 10 includes a chamber 11 for containing laser gasin a sealed condition, a band narrowing module 14 for forming anoscillator and a partial reflection mirror 15 that operates as an outputmirror. It additionally includes a laser gas exciting system and acontrol system, which are not represented, as well as a cooling systemand a gas exchange system.

Two windows 12 and 13 are arranged on the optical axis L in the chamber11. The band narrowing module 14 includes a single beam expanding prismor a plurality of beam expanding prisms 16 (two in FIG. 12) for forminga beam expanding optical system and a grating 17 (or an etalon) thatoperates as a band narrowing element.

The amplifier laser 20 also includes a chamber 21 for containing lasergas in a sealed condition and partial reflection mirrors 24, 25 forforming an oscillator. It additionally includes a laser gas excitingsystem and a control system, which are not illustrated, as well as acooling system and a gas exchange system.

Two windows 22 and 23 are arranged on the optical axis L in the chamber21. Referring to FIG. 12, the laser beam oscillated by the oscillatorlaser 10 is reflected by the mirrors 18 and 19 to enter the amplifierlaser 20.

The laser beam that exits from the partial reflection mirror 25 thenpasses an optical pulse stretcher 50 and enters a degree of polarizationcontrol device 30.

The degree of polarization control device 30 includes a CaF₂ crystalsubstrate 31, a rotary stage 32 to which the CaF₂ crystal substrate 31is fitted, the rotary stage 32 and the CaF₂ crystal substrate 31 forminga control module 33, a beam splitter 34 for taking out part of the laserbeam at a position downstream relative to the CaF₂ crystal substrate 31,a polarization monitor 35 for receiving the laser beam taken out fromthe beam splitter 34 and measuring the degree of polarization of thelaser beam, a controller 36 for computationally determining if controlis required or not from the output signal on the degree of polarizationmeasured by the polarization monitor 35 and, if necessary, determiningthe extent of rotation of the CaF₂ crystal substrate 31 and a rotarydriver 37 for driving the rotary stage 32 in response to the extent ofrotation as computationally determined by the controller 36.

The CaF₂ crystal substrate 31 is formed by a parallel plate whosesurface is polished at the (111) crystal face so as to make the incidentangle be Brewster angle and that can rotate at the [111] axis, theparallel plate being arranged on the laser optical axis extending fromthe PO laser 20.

One of the techniques disclosed in JP-A-2007-214189 is preferablyapplied to the polarization monitor 35. For example, as illustrated inFIG. 13, a substrate prepared by arranging a ps separation membrane 35 bon a polarization monitoring CaF₂ crystal substrate 35 a is employed tomeasure the p-polarized component and the s-polarized component of thelaser beam. As the degree of polarization is degraded, the s-polarizedcomponent increases so that the degree of polarization can be measuredby determining the ratio of the s-polarized component.

Now, a control flowchart, or control flowchart Example 1 of the degreeof polarization control device 30 of the first embodiment will bedescribed below. FIG. 14 is a control flowchart illustrating the degreeof polarization control device 30 of Example 1.

Firstly, in Step 1, the desired degree of polarization PO and thethreshold value Pth for the degree of polarization on which a controloperation is to be executed are defined as parameters (ST 1). Then, inStep 2, it is determined if the number of measurement shots is equal tothe predetermined number or not for checking the degree of polarizationwhile the laser is in operation (ST 2).

If it is determined in Step 2 that the number of measurement shots isnot equal to the predetermined number, the control operation returns toStep 2. If, on the other hand, it is determined in Step 2 that thenumber of measurement shots is equal to the predetermined number, thedegree of polarization P is measured by the polarization monitor 35 inStep 3 (ST 3).

Then, in the next step, or Step 4, it is determined if the degree ofpolarization P measured in Step 3 is smaller than the threshold valuePth defined in Step 1 or not (ST 4).

If it is determined in Step 4 that the degree of polarization P is notsmaller than the threshold value Pth, the control operation returns toStep 2. If, on the other hand, it is determined in Step 4 that thedegree of polarization P is smaller than the threshold value Pth, thecurrent degree of polarization P, the desired degree of polarization POand the rotation angle θa of the control module 33 are determined inStep 5, the rotation angle θa of the control module 33 being determinedfrom the predetermined degree of polarization table on the basis of thecurrent rotational position θ of the CaF₂ crystal substrate 31 (ST 5).Note, however, that a degree of polarization table depicts therelationship between the rotational angle and the degree of polarizationof the incident laser beam after being transmitted through the crystalas seen from FIGS. 9 and 10. Thus, the rotational angle θa for achievingthe desired degree of polarization PO is determined from the table. Alsonote that the degree of polarization table varies from element toelement.

In the next step, or Step 6, a signal is transmitted to the rotarydriver 37 to drive the rotary stage 32 of the CaF₂ crystal substrate 31to rotate by dθ (dθ=θa−θ) from the current position (ST 6). At thistime, θ and n are turned respectively to θ=θa and n=n+1.

In the next step, or Step 7, it is determined if the number of times nof repeating the control routine exceeds the predefined number of timesnth and hence n>nth or not (ST 7). If it is determined in Step 7 thatthe number of times n of repeating the control routine does not exceedthe predefined number of times nth and hence not n>nth, the controloperation returns to Step 3. If, on the other hand, it is determined inStep 7 that the number of times n of repeating the control routineexceeds the predefined number of times nth and hence n>nth, abnormaldegree of polarization is determined and an abnormal signal is output toend the control operation.

Now, a control flowchart, or control flowchart Example 2 of the degreeof polarization control device 30 of the first embodiment will bedescribed below. FIG. 15 is a control flowchart illustrating the degreeof polarization control device 30 of Example 2.

Firstly, in Step 11, the desired degree of polarization PO and thethreshold value Pth for the degree of polarization on which a controloperation is to be executed are defined as parameters (ST 11). Then, inStep 12, it is determined if the number of measurement shots is equal tothe predetermined number or not for checking the degree of polarizationwhile the laser is in operation (ST 12).

If it is determined in Step 12 that the number of measurement shots isnot equal to the predetermined number, the control operation returns toStep 12. If, on the other hand, it is determined in Step 12 that thenumber of measurement shots is equal to the predetermined number, thedegree of polarization P is measured by the polarization monitor 35 inStep 13 (ST 13). Note that dP=P0−P.

Then, in the next step, or Step 14, it is determined if the degree ofpolarization P measured in Step 13 is smaller than the threshold valuePth defined in Step 11, or P<Pth, or not (ST 14).

If it is determined in Step 14 that the degree of polarization P is notsmaller than the threshold value Pth, the control operation returns toStep 12. If, on the other hand, it is determined in Step 14 that thedegree of polarization P is smaller than the threshold value Pth, asignal is transmitted to the rotary driver 37 in Step 15 to drive therotary stage 32 of the CaF₂ crystal substrate 31 to rotate by +kdθobtained by multiplying predetermined micro rotational angle dθ bycoefficient k (ST 15). At this time, n is turned to n=n+1.

Then, in the next step, or Step 16, the degree of polarization Pn ismeasured again by the degree of polarization monitor 35 after therotation (ST 16). At this time, dPn is turned to dPn=P0−Pn.

Then, in the next step, or Step 17, it is determined if dPn<dP holdstrue or not according to the measured degree of polarization (ST 17). Ifit is determined in Step 17 that dPn<dP does not hold true, the measureddegree of polarization is moving away from the desired degree ofpolarization P0 because the rotational direction is opposite so that kis turned to k=k·(−1) and the control operation returns to Step 15.

If, on the other hand, it is determined in Step 17 that dPn<dP holdstrue, it is then determined in Step 18 if the degree of polarization hasgot into a range that takes the tolerance value Pa into consideration ornot, or if dPn<P0±Pa holds true or not (ST 18). If it is determined inStep 18 that dPn<P0±Pa holds true, the control operation returns to Step12.

If, on the other hand, it is determined in Step 18 that dPn<P0±Pa doesnot hold true, it is determined in Step 19 if the number of times n ofrepeating the control routine exceeds the predefined number of times nthand hence n>nth or not (ST 19).

If it is determined in Step 19 that the number of times n of repeatingthe control routine does not exceed the predefined number of times nthand hence not n>nth, the control operation returns to Step 15. If, onthe other hand, it is determined in Step 19 that the number of times nof repeating the control routine exceeds the predefined number of timesnth and hence n>nth, it is then determined in Step 20 if the degree ofpolarization Pn is smaller than the threshold value Pth, or Pn<Pth, ornot (ST 20). If it is determined in Step 20 that Pn<Pth holds true, thecontrol operation returns to Step 12. If, on the other hand, it isdetermined in Step 20 that Pn<Pth does not hold true, an abnormal signalis output to end the control operation.

Now, the second embodiment will be described below. The degree ofpolarization control device 30 of the second embodiment has a pluralityof CaF₂ crystal substrates 31. For example, as illustrated in FIG. 16,the degree of polarization control device 30 has a first CaF₂ crystalsubstrate 31 a, a first rotary stage 32 a, the first CaF₂ crystalsubstrate 31 a and the first rotary stage 32 a forming a first controlmodule 33 a, and a first rotary driver 37 a as well as a second CaF₂crystal substrate 31 b, a second rotary stage 32 b, the second CaF₂crystal substrate 31 b and the second rotary stage 32 b forming a secondcontrol module 33 b, and a second rotary driver 37 b. Otherwise, thesecond embodiment has a configuration similar to the first embodimentand hence will not be described any further.

When the degree of polarization control device 30 of the secondembodiment cannot improve the degree of polarization to a desired levelby controlling the first control module 33 a that includes the firstCaF₂ crystal substrate 31 a, it starts controlling the second controlmodule 37 b that includes the second CaF₂ crystal substrate 31 b torestore the desired level of degree of polarization. Preferably, itinitially controls only the first CaF₂ crystal substrate 31 a and doesnot control the second CaF₂ crystal substrate 31 b, holding θ=0. Thenumber of CaF₂ crystal substrates is not limited to two and more thantwo CaF₂ crystal substrates may be provided and controlled sequentially.If such is the case, the Brewster angle that makes the reflectance equalto 0 for the p-polarized component is selected for the incident angle inorder to prevent any loss from arising on the substrate surface.

Now, a control flowchart, or control flowchart Example 1 of the degreeof polarization control device 30 of the second embodiment will bedescribed below. FIG. 17 is a control flowchart, or control flowchartExample 1, of the degree of polarization control device 30.

Firstly, in Step 21, the desired degree of polarization PO and thethreshold value Pth for the degree of polarization on which a controloperation is to be executed are defined as parameters (ST 21). Then, inStep 22, it is determined if the number of measurement shots is equal tothe predetermined number or not for checking the degree of polarizationwhile the laser is in operation (ST 22).

If it is determined in Step 22 that the number of measurement shots isnot equal to the predetermined number, the control operation returns toStep 22. If, on the other hand, it is determined in Step 22 that thenumber of measurement shots is equal to the predetermined number, thedegree of polarization P is measured by the polarization monitor 35 inStep 23 (ST 23). Note that dP=P0−P.

Then, in the next step, or Step 24, it is determined if the degree ofpolarization P measured in Step 23 is smaller than the threshold valuePth and hence P<Pth defined in Step 21 or not (ST 24).

If it is determined in Step 24 that the degree of polarization P is notsmaller than the threshold value Pth, the control operation returns toStep 22. If, on the other hand, it is determined in Step 4 that thedegree of polarization P is smaller than the threshold value Pth, thecurrent degree of polarization P, the desired degree of polarization POand the rotation angle θa of the control module 33 are determined inStep 25, the rotation angle θa of the control module 33 being determinedfrom the predetermined degree of polarization table on the basis of thecurrent rotational position θ of the CaF₂ crystal substrate 31 (ST 25).Note, however, that a degree of polarization table depicts therelationship between the rotational angle and the degree of polarizationof the incident laser beam after being transmitted through the crystalas seen from FIGS. 9 and 10. Thus, the rotational angle θa for achievingthe desired degree of polarization P0 is determined from the table. Alsonote that the degree of polarization table varies from element toelement.

In the next step, or Step 26, a signal is transmitted to the rotarydriver 37 to drive the rotary stage 32 of the CaF₂ crystal substrate 31to rotate by dθ (dθ=θa−θ) from the current position (ST 26). At thistime, θ and n are turned respectively to θ=θa and n=n+1.

In the next step, or Step 27, it is determined if the number of times nof repeating the control routine exceeds the predefined number of timesnth and hence n>nth or not (ST 7).

If it is determined in Step 27 that n>nth holds true, then it isdetermined in Step 28 if the number of control modules is m=mmax andhence all the modules have been controlled or not (ST 28). If it isdetermined in Step 28 that m=mmax holds true, an abnormal signal isoutput to end the control operation. If, on the other hand, it isdetermined in Step 28 that m=mmax does not hold true, m and n are turnedrespectively to m=m+1 and n=0 and the control operation returns to Step22.

Now, a control flowchart, or control flowchart Example 2 of the degreeof polarization control device 30 of the second embodiment will bedescribed below. FIG. 18 is a control flowchart, or control flowchartExample 2, of the degree of polarization control device 30.

Firstly, in Step 31, the desired degree of polarization PO and thethreshold value Pth for the degree of polarization on which a controloperation is to be executed are defined as parameters (ST 31). Then, inStep 32, it is determined if the number of measurement shots is equal tothe predetermined number or not for checking the degree of polarizationwhile the laser is in operation (ST 32).

If it is determined in Step 32 that the number of measurement shots isnot equal to the predetermined number, the control operation returns toStep 32. If, on the other hand, it is determined in Step 32 that thenumber of measurement shots is equal to the predetermined number, thedegree of polarization P is measured by the polarization monitor 35 inStep 33 (ST 33). Note that dP=P0−P.

Then, in the next step, or Step 34, it is determined if the degree ofpolarization P measured in Step 33 is smaller than the threshold valuePth defined in Step 31, or P<Pth, or not (ST 34).

If it is determined in Step 34 that the degree of polarization P is notsmaller than the threshold value Pth, the control operation returns toStep 32. If, on the other hand, it is determined in Step 34 that thedegree of polarization P is smaller than the threshold value Pth, asignal is transmitted to the rotary driver 37 in Step 35 to drive therotary stage 32 of the CaF₂ crystal substrate 31 to rotate by +kdθobtained by multiplying predetermined micro rotational angle dθ bycoefficient k (ST 35). At this time, n is turned to n=n+1.

Then, in the next step, or Step 36, the degree of polarization Pn ismeasured again by the degree of polarization monitor 35 after therotation (ST 36). At this time, dPn is turned to dPn=P0−Pn.

Then, in the next step, or Step 37, it is determined if dPn<dP holdstrue or not according to the measured degree of polarization (ST 37). Ifit is determined in Step 37 that dPn<dP does not hold true, the measureddegree of polarization is moving away from the desired degree ofpolarization P0 because the rotational direction is opposite so that kis turned to k=k·(−1) and the control operation returns to Step 35.

If, on the other hand, it is determined in Step 37 that dPn<dP holdstrue, it is then determined in Step 38 if the degree of polarization hasgot into a range that takes the tolerance value Pa into consideration ornot, or if dPn<P0±Pa holds true or not (ST 38). If it is determined inStep 38 that dPn<P0±Pa holds true, the control operation returns to Step32.

If, on the other hand, it is determined in Step 38 that dPn<P0±Pa doesnot hold true, it is determined in Step 39 if the number of times n ofrepeating the control routine exceeds the predefined number of times nthand hence n>nth or not (ST 39).

If it is determined in Step 39 that the number of times n of repeatingthe control routine does not exceed the predefined number of times nthand hence not n>nth, the control operation returns to Step 35. If, onthe other hand, it is determined in Step 39 that the number of times nof repeating the control routine exceeds the predefined number of timesnth and hence n>nth, it is then determined in Step 40 if the degree ofpolarization Pn is smaller than the threshold value Pth, or Pn<Pth, ornot (ST 40).

If it is determined in Step 40 that Pn<Pth holds true, the controloperation returns to Step 32. If, on the other hand, it is determined inStep 40 that Pn<Pth does not hold true, then it is determined in Step 41if the number of control modules is m=mmax and hence all the moduleshave been controlled or not (ST 41).

If it is determined in Step 41 that m=mmax holds true, an abnormalsignal is output to end the control operation. If, on the other hand, itis determined in Step 41 that m=mmax does not hold true, m and n areturned respectively to m=m+1 and n=0 and the control operation returnsto Step 33.

Now, a third embodiment of the present invention will be describedbelow. The 2-stage laser system 1 of the third embodiment is providedwith a monitor module 60 for measuring the laser output energy level,the spectrum width and the center wavelength as illustrated in FIG. 19,for instance. Additionally, the CaF₂ crystal substrate 31 of the degreeof polarization control device 30 is made to take the role of a beamsplitter for entering light into the monitor module 60. In this case,the incident angle is 45° but the characteristics for shifting thedegree of polarization of laser beam is the same as the case of usingthe Brewster angle as an incident angle. Otherwise, the third embodimenthas a configuration similar to the first embodiment and hence will notbe described any further.

As the CaF₂ crystal substrate 31 of the degree of polarization controldevice 30 is made to take the role of a beam splitter for the monitormodule 60, this embodiment has an advantage of reduced cost and areduced space it requires. Instead of providing a beam splitter for themonitor module 60, a rotary mechanism for controlling polarization maybe annexed to the transmission element arranged on the main optical axisof the system to achieve the same effect.

The quantity of variation of the phase difference due to transmissionthrough the CaF₂ crystal substrate varies as a function of thermalstress when it is rigidly secured at a certain rotation angle θ.Therefore, when a command for shifting the energy level and thefrequency is received, the rotational angle θ is preferably adjusted atthe same time in order not to change the degree of polarization.

Furthermore, the quantity of variation of the phase difference alsovaries when the rotational angle is fixed and the incident angle isshifted. Therefore, it is also possible to control the incident angleinstead of the rotational angle in order to control the degree ofpolarization. However, if such is the case, the optical axis will beshifted after the transmission so that preferably a compensation elementis inserted before the laser beam enters an aligner in order to preventthe optical form being shifted and the compensation element iscontrolled at the same time. The rotational angle thereof is preferablyset to θ=0° so that the degree of polarization may not vary as a resultof transmission through the compensation element.

The quantity of variation of the phase difference as a result oftransmission through the CaF₂ crystal substrate 31 varies according tothe producer and the grade of the element. Therefore, an optimum elementthat is within the range to be desirably controlled is preferablyselected.

While CaF₂ crystal cut along the (111) crystal face is employed in theabove description, it is easily predictable that the degree ofpolarization will be shifted by using an element cut along some othercrystal orientation and modifying the relationship between the opticalaxis of transmission and the crystal orientation. Therefore, when anelement cut along some other crystal orientation is employed, thecharacteristics that become apparent when it is driven to rotate may beobserved and the element may be controlled in a manner that is congruouswith the characteristics.

While a MOPO type laser is described above, the present invention isequally applicable to lasers other than the MOPO type such as MOPA typelasers and ring amplification type lasers to provide similar advantages.

Additionally, while the present invention is applied to the optical axisdownstream relative to an amplification stage in the above description,it is also possible to arrange a polarization control module 33 and apolarization monitor 35 downstream relative to an oscillation stagelaser to control the degree of polarization of the seed laser beam.

Furthermore, while the present invention is described above in terms ofCaF₂ crystal, MgF₂ crystal provides similar effects and hence the degreeof polarization can be controlled by controlling the relationshipbetween the optical axis of a laser beam transmitted through MgF₂crystal and the crystal orientation.

While the present invention is described in terms of embodiments ofdegree of polarization control device 30, the present invention is by nomeans limited to those embodiments, which may be modified in variousdifferent ways without departing from the spirit and scope of theinvention.

1. A degree of polarization control device comprising: a calciumfluoride crystal substrate for transmitting a laser beam; a polarizationmonitor for measuring the degree of polarization of a laser beamtransmitted through the calcium fluoride crystal substrate; and acontroller for controlling the rotation angle of the calcium fluoridecrystal substrate according to the degree of polarization measured bythe polarization monitor, wherein the calcium fluoride crystal substratebeing formed by a flat plate having a laser beam entering surface and alaser beam exiting surface running in parallel with the (111) crystalface, the Brewster angle being selected for the incident angle, therotation angle around the (111) axis operating as a central axis beingcontrolled by the controller.
 2. The degree of polarization controldevice according to claim 1 comprising a plurality of calcium fluoridecrystal substrates.
 3. The degree of polarization control deviceaccording to claim 1 further comprising: a monitor module for observingthe condition of the laser beam; the calcium fluoride crystal substratebeing adapted to split the laser beam in order to make it enter themonitor module.
 4. A gas laser apparatus provided with a degree ofpolarization control device according to claim 1, the apparatuscomprising: a chamber; a laser gas contained in the chamber in a sealedcondition; means for exciting the laser gas; a window arranged at thechamber to allow the laser beam generated from the excited laser gas toexit to the outside of the chamber; and a degree of polarization controldevice for controlling the degree of polarization of the laser beamexiting from the window.
 5. A gas laser apparatus provided with a degreeof polarization control device according to claim 2, the apparatuscomprising: a chamber; a laser gas contained in the chamber in a sealedcondition; means for exciting the laser gas; a window arranged at thechamber to allow the laser beam generated from the excited laser gas toexit to the outside of the chamber; and a degree of polarization controldevice for controlling the degree of polarization of the laser beamexiting from the window.
 6. A gas laser apparatus provided with a degreeof polarization control device according to claim 3, the apparatuscomprising: a chamber; a laser gas contained in the chamber in a sealedcondition; means for exciting the laser gas; a window arranged at thechamber to allow the laser beam generated from the excited laser gas toexit to the outside of the chamber; and a degree of polarization controldevice for controlling the degree of polarization of the laser beamexiting from the window.